The human monocyte possesses a specific phagocytic receptor for particulate activators of the human alternative complement pathway. The selective interaction of a nonimmune 180,000 Mr fragment of human plasma fibronectin and particulate activators results in their augmented phagocytosis by human monocytes. The mechanism of enhanced phagocytosis provided by the 180,000 Mr fragment involves its selective binding to particulate activators and linking them to monocyte fibronectin adherence receptors, which promotes their ingestion through the receptor for particulate activators. This proposal deals with the structure, function, and generation of the 180,000 Mr fragment. Research will be directed first, at mapping the 180,000 Mr fibronectin fragment within native fibronectin of Mr 440,000 with monoclonal antibodies to fibronectin and at analyzing this fragment for three functions of native fibronectin, second, at localizing the two functional domains within the 180,000 Mr fragment that mediate binding to particulate activators and to human monocyte fibronectin receptors, respectively, with Fab fragments of the monoclonal anti-fibronectin antibodies, third, at developing monoclonal anti-idiotypes to the site on the target activator and to the fibronectin receptors with the monoclonal anti-fibronectin that directly block interaction of the 180,000 Mr fragment at these two respective sites, and finally, at generating the phagocytically active fragment with various intravascular proteolytic systems. Long-term objectives are first, to prepare definitive monoclonal immunoreagents for detection of total and selected forms of human fibronectin, second, to analyze biochemically a defined opsonin with respect to its target and monocyte binding domains, third, to isolate and/or characterize the activating principle on particulate activators and the monocyte fibronectin receptors, and finally to identify a potential in vivo source of generating the opsonic fragment. The identification of such a proteolytic system would complete a novel system of nonimmune host defense which involves the 180,000 Mr fibronectin fragment with its marked specificity for particulate activators and two specific human monocyte receptors, one for the adherence domain of the opsonic fragment and the other for phagocytosis of particulate activators.